Energetic protons in the Jovian magnetosphere

Abstract

The time histories, angular distributions and energy spectra of energetic protons have been measured over an energy range extending from 0.2 to 20 MeV for the four passes of Pioneers 10 and 11 through the Jovian magnetosphere. The energetic particle data from these four passes are remarkably different. Azimuthal asymmetries appear to dominate with time variations also contributing to the very complex topology. On the inbound P‐10 pass the expected corotation anisotropy was not observed in the outer magnetosphere. The simplest explanation is that the particle reference frame (i.e., the magnetospheric plasma in this region) is moving nearly radially, suggesting the existence of a planetary wind at that time. Near the dawn meridian, particle streaming away from the planet begins at ∼15 RJ. On both the P‐10 inbound and P‐11 outbound passes, there are regions where only partial corotation is achieved. In the mid‐magnetosphere, field‐aligned streaming away from the near‐equatorial current sheet region is the most prominent and puzzling feature. At mid‐latitudes in the subsolar regime, the streaming pattern is more chaotic and its magnitude is smaller. Qualitative discussions are presented for a number of possible mechanisms which could produce this streaming. In the context of our present understanding of the Jovian magnetosphere, each of three mechanisms show promise: perpendicular electric fields, a strong azimuthal intensity gradient at the equator, and strong diffusion processes. In the Jovian wind regions, proton energy spectra are generally of the form E−γ, where E is the kinetic energy of the particle with γ generally ranging between values of 3.0 and 4.2. For the remaining times, the spectra are most frequently of the form exp −P/P0, where P is the proton momentum with P0 varying mainly between 8.0 and 12 MV.